1. Field of the Invention
The present disclosure relates to an imaging optical system (wide-angle imaging lens system) that can be installed in an imaging apparatus such as, e.g., an on-board camera, a surveillance camera, and a camera in a portable terminal.
2. Description of Related Art
In recent years, devices that use a camera mounted on or in a motor vehicle to recognize surrounding objects, apply an automatic brake if a collision is anticipated, or to follow a vehicle traveling in front of the motor vehicle have become commonplace. There is a demand for such on-board cameras to not only have surveillance capabilities in front of the vehicle, but also surveillance capabilities for a wider range in forward diagonal directions in which sidewalks (footpaths) and intersections can be captured. A wide-angle lens system is used to carry out surveillance over a wider range; however, a typical (conventional) wide-angle lens system has a small magnification, which risks a reduction in object recognition precision. In particular, since a high detection precision is demanded at a central area in which a vehicle in front or an on-coming vehicle is located, it is preferable to obtain a large image. Therefore, there is a demand for the on-board camera to be capable of capturing (shooting) a wide range/area while capturing a large image at the central area of the image.
Japanese Unexamined Patent Publication No. 2004-354572 (herein “Patent Publication 1”) discloses an imaging apparatus provided with a compact image input optical system that employs a projection method that enlarges the object image at the central area of the picture screen and does not need a complex arrangement or configuration such as a zoom optical system or a superwide angle lens system. Japanese Unexamined Patent Publication No. 2005-10521 (herein “Patent Publication 2”) discloses an imaging apparatus provided with a large-aperture, compact, high quality imaging lens that utilizes the advantages of the fovea lens.
However, in the imaging apparatuses disclosed in Patent Publications 1 and 2, the diameter of the first lens provided closest to the object side is large, and the degree of retroflection on the peripheral profile on the surface on the object side of the first lens is large. Since the on-board camera (optical system) is typically provided at the top of the front windscreen of the motor vehicle at a position very close to the inner side of the front windshield (windscreen), if the diameter and the retroflection of the first lens are large, the lens unit increases in size and thereby obstructs the view of the driver of the motor vehicle and clutters the spatial interior of the motor vehicle.